This is a good topic because I wonder why, when a station makes a dramatic and unexpected shift, such as AC to Active Rock, how are there advertisers on Day One? Is it that some advertisers don't care? They buy 52 week schedules and even if the new format is someone reading the phone book, if the rate is low enough, they are on it?
Is the station practically giving away the spots since they don't want to sound like nobody is advertising? I worked for a station that was running a network format. To make it sound like advertisers were interested in us, we ran network spots in all available breaks, even if we weren't getting paid for them. Of course, some music stations do a promotion like 40,000 songs in a row. I remember a number of stations have done this, including KCYY San Antonio, when it switched to country music in the 1990s.
If the format flip is unannounced, as David says above, you can't tell your advertisers more than a couple of days in advance because they will SURELY tell your competitor stations.